Transesterification reactions are a commercially significant class of industrial organic reactions. In a transesterification reaction an ester is converted to another ester by exchange of the acid groups or by exchange of the alcohol groups. If the transesterification takes place by exchange of the alcohol groups, it is referred to as alcoholysis. In alcoholysis the alcohol to be exchanged is generally added in excess in order to give a high yield of the desired ester. Recently, in connection with the production of diesel fuel from renewable resources, the preparation of alkyl esters, especially methyl esters from vegetable oils (e.g. rapeseed oil, soya bean oil), has acquired considerable relevance.
Transesterification is an equilibrium reaction that is usually initiated simply by mixing the reactants. The reaction proceeds so slowly, however, that for commercial purposes a catalyst is required to accelerate the reaction. Fats and oils of biological origin consist predominantly of glycerides (mono-, di- and triglyceride). In practice the Bradshaw process is often used for the transesterification of fats and oils with methanol (described in U.S. Pat. Nos. 2,271,619 and 2,360,844). The reaction is carried out in an open vessel, which can be made of ordinary carbon steel. In this process the fat or oil, whose acid number should not exceed 1.5, is stirred at the boiling point of the reaction mixture with an excess of 99.7% methanol in the presence of 0.1 to 0.5% of sodium hydroxide. On standing, the glycerol formed separates out at the bottom of the vessel in virtually anhydrous form. After one hour the conversion is usually 98%. The fat or oil must be dry (anhydrous), clean and, in particular, neutral.
If sodium and potassium compounds are used as catalysts in the transesterification of triglycerides with methanol and ethanol, various problems arise. After partial completion of the transesterification reaction, the glycerol produced begins to form a new phase. The sodium and potassium compounds used as catalysts are very readily soluble in the glycerol phase and are accordingly depleted in the reaction mixture. For this reason, and because of the emulsion formed in the course of the reaction, the reaction ultimately progresses only slowly, which is why the reaction is often stopped after about half an hour to separate off the glycerol formed. Fresh catalyst is then added and the reaction is continued for a further half an hour. A conversion of about 98% is obtained in this way. Very fine glycerol droplets still remain suspended in the ester phase after the phase separation. The glycerol and the catalyst partitioned between the two phases must be removed from the ester phase when the reaction has ended. Depending on the subsequent use of the glycerol, it is further necessary also to remove the dissolved catalyst from the glycerol phase. Various suggestions have been made to avoid said problems and simultaneously shorten the reaction time.
Published German patent application DE 34 21 217 A1 describes a process for the preparation of fatty acid esters of short-chain primary and secondary alcohols having 1 to 5 carbon atoms by the transesterification of glycerides, wherein a stream of the gaseous alcohol is passed through the liquid glycerides at temperatures of between 230 and 270° C. A product mixture of glycerol and fatty acid alkyl ester is entrained with this stream out of the reaction zone and is then separated. Alkali is dissolved as catalyst in the liquid glycerides contained in the reaction vessel.
In the process described in German patent DE 198 03 053 C1, triglycerides are reacted with several times the molar amount of alcohol in the presence of suitable catalysts, e.g. zinc soaps, preferably in cocurrent columns at temperatures of 200 to 240° C. and at pressures of up to 90 bar. After the excess alcohol has been separated off, the alkyl ester/glycerol mixtures obtained are separated into the lighter organic phase and the glycerol phase in a separator. This phase separation is followed by a further work-up and purification of the products. The ester phase is washed with water to remove the glycerol residues dissolved in the product. About 40% of the zinc soaps dissolved in the ester is also washed out in the form of zinc hydroxide during this step.
There have also been experiments aimed at replacing the sodium and potassium compounds with basic ammonium compounds as catalysts or reactants. The activity of numerous bases has been studied with regard to their suitability as catalysts for the alcoholysis of fats and oils, examples of said bases being amines such as triethylamine, 1,2,2,6,6-pentamethylpyridine, 2,6-ditert-butylpyridine and 4-dimethylaminopyridine (DMAP); 1,5,7-triazabicyclo(4.4.0)dec-5-ene (TBD), 1,1,3,3-tetramethylguanidine (TMG), 1,2,3-triphenylguanidine, 1,1,2,3,3-pentabutylguanidine (PBG), 1,3-diphenylguanidine and other aminoguanidines and nitroguanidines; and triamino(imino)phosphoranes such as tert-butylamino-2-diethylamino-1,3-perhydro-1,2,3-diazaphorane (BEMP) and tris(dimethylamino)methyliminophosphorane (Me7P). The latter are frequently used in organic synthesis. In one series the catalytic activity of some guanidine compounds, e.g. the amidines DBU and DBN, and the phosphoranes BEMP and Me7P, was compared with that of other bases. The guanidines are the more active catalysts, the activity following their relative basicity. The activity of TBD at a concentration of 3 mol % was similar to that of potassium carbonate at the same concentration.
One advantage offered by guanidines as catalysts in the transesterification of fats and oils is the possibility of binding them to organic polymers. Schuchardt et al. studied the suitability of cellulose, polystyrene/divinylbenzene and polyurethanes as supports. The preparation of heterogeneous catalysts by binding guanidines to various polymers by means of chemical binding forces, and their suitability for catalysing the transesterification of vegetable oils and fats, are described in Brazilian patent BR 8202429 (1984, inventors: U. Schuchardt and O. G. Lopes). The guanidines bound to gelatinous poly(styrene/divinylbenzene) or cellulose showed a slightly reduced activity compared with the catalytic reaction in the homogeneous phase. After prolonged reaction times, however, the same conversion levels were achieved. Although somewhat less active than the analogous homogeneous catalysts, the heterogeneous catalysts could be re-used in a number of consecutive reaction cycles, albeit with a noticeable drop in activity after only 9 reaction cycles. The drop in activity with continuous use was caused by the slow leaching of the anchored base out of the polymer.
Published international patent application WO 2004/031119 describes the use of basic catalysts selected from imino compounds, alkylguanidines, butylamine, quaternary amines and tertiary amines carrying an additional OH group or NH2 group.
In published international patent application WO 01/12581, it is proposed in a first step to deacidify the vegetable oil or fat by reacting the free fatty acids with methanol in the presence of sulfuric acid as catalyst. In a second step, after neutralization, transesterification is carried out using alkali (NaOH, KOH). In the second step, a cosolvent is mixed with the reaction mixture in an amount such that the latter becomes a single phase, thereby considerably increasing the reaction rate. Tetrahydrofuran, methyl tert-butyl ether, pyridine and bis-(dimethylsilyl)trifluoroacetamide are mentioned as suitable cosolvents.